Desirable aspects for a radio frequency (RF) MEMS electrostatic tunable capacitor include, but are not limited to, the ability to be used in shunt or series configurations, high electrical quality factor, low sensitivity to external packaging effects, low sensitivity to temperature variations, low sensitivity to process variations, low actuation voltage, resistance to self-actuation due to applied RF energy, and large elastic restoring force in order to minimize stiction reliability concerns.
Cantilever beam actuator designs are good for addressing and minimizing external effects from packaging stresses, especially over temperature by isolating the MEMS beam element from the attachment substrate, but such structures can be sensitive to fabrication process variations, such as stress gradients which affect beam shape and relative displacement. Specifically, for example, they tend to be sensitive to fabrication process variations that influence the stress gradient through the beam, which in turn affect beam deflection in a negative fashion, particularly at the beam tip.
In addition, the typical electrostatic cantilever beam switch design has the signal capacitor head located at the distal end of the beam beyond the actuator motor region, which in turn makes it difficult to minimize the actuation pull-in voltage while at the same time trying to maximize the elastic restoring force in the beam such that the beam will return to its initial open un-actuated state when desired since the two have such tightly-coupled electro-mechanical behavior. Further, compared to a nominally flat coplanar MEMS cantilever beam structure, a cantilever beam deflected out-of-plane away from the substrate will require higher applied voltages for electrostatic actuation, which is usually an undesirable trait. A beam deflected towards the substrate will have undesirable traits of a lower self-actuation voltage and less restoring force available for overcoming adhesion forces in the actuated state.
Alternatively, multi-support MEMS switchable structures can address some of these issues of cantilever beams, but they can also introduce new problems. For instance, multi-support MEMS switchable structures often require narrow and/or folded support structures in order to minimize influences from residual stresses, thermal stresses, and substrate effects on the structural shape. Such multiple narrow folded support structures, while good for mechanical considerations, are not conducive to good electrical design considerations of minimizing insertion loss for RF signals routed onto the MEMS element structure. This is primarily due to the inherent increase in electrical path resistance and inductance of narrow folded support beams when compared to an RF signal running onto a straight wide cantilever beam.
As a result, it would be beneficial to decouple mechanical design from electrical design such that each aspect can be designed and optimized in a more independent fashion to meet performance, reliability, and cost objectives, as well as providing robustness to process and temperature variation.